Maintaining the integrity of sewer collection systems has been a significant challenge for managing municipalities and utilities since the use of such systems began. Increased demand on the systems by growing user populations as well as dilapidation due to aging and chemical corrosion continuously create leaks in the system components.
During periods of ground saturation, such as during heavy rain storms, the ground water can infiltrate the sewer collection system through these breaches. The increased water in the system adds to the flow that must be processed by the system's water treatment plant. This increased load can often exceed the capacity for the treatment plant, and create further complications and possible damage to the overall system. Additionally, breaks in the sewer lines can permit leakage of sewage from the system into the surrounding soil. This contamination is unacceptable, particularly in view of ever increasing concerns for environmental safety.
One common component of sewer collection systems that is often in need of rehabilitation is the manhole. Manholes provide human access ports from the grade or street level to open pipeline "inverts" for inspection and repair of the sewer collection system. Manholes are not only exposed to sewage flow within the system but also to run-off from above. Manholes are typically made from precast concrete, brick, tiles or concrete blocks bonded together by mortar. These construction elements can be deteriorated by chemical attack, such as by sulfuric acid microbiologically generated by the hydrogen sulfide present in the sewage. The corroded materials can break away, crack or separate, thereby creating breaches through which leakage and seepage can occur.
Many techniques for the rehabilitation of deteriorated manholes have been developed. Ruptures, particularly those causing large leaks, are patched using grouting. Hydrophilic gel is a water-activated grout solution that can fill and seal cracks and joints with a flexible mass. The gel can also be used to fill water collecting voids between the leaking manhole and the surrounding soil. The gel can further penetrate and consolidate the surrounding soil to produce a substantially water impermeable grout-soil mass or grout membrane. While grouting is effective in reducing relatively large leak flows, it is too expensive to use as a sealant for the entire manhole structure and is difficult to control because of its injection through the manhole wall to the otherwise inaccessible, outer "positive" side.
For structurally rehabilitating and chemically safeguarding an entire manhole surface area, Portland cement or calcium aluminate cement are typically used. While these materials have superior chemical resistance properties, they can crack due to ground shifts and other disturbances and permit leakage in and out of the system.
Epoxy coatings in the form of sleeve-like liners or spray or hand-applied coatings also provide resistance against attack by a variety of corrosive chemicals as well as abrasion. However, these epoxy surfaces are susceptible to rupture and peeling when hydrostatically pressured from behind by external water infiltration through cracks in the underlying manhole walls.
While the various techniques and materials for rehabilitating manholes provide advantages individually, the associated disadvantages of these techniques make them inappropriate individually as long-term solutions to establishing and maintaining manhole integrity.